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std::bad_expected_access

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Defined in header <expected>
template< class E >
class bad_expected_access : public std::bad_expected_access<void>
(1) (since C++23)
template<>
class bad_expected_access<void> : public std::exception
(2) (since C++23)
1) Defines a type of object to be thrown by std::expected::value when accessing an expected object that contains an unexpected value. bad_expected_access<E> stores a copy of the unexpected value.
2) bad_expected_access<void> is the base class of all other bad_expected_access specializations.
All member functions of std::bad_expected_access are constexpr: it is possible to create and use std::bad_expected_access objects in the evaluation of a constant expression.

However, std::bad_expected_access objects generally cannot be constexpr, because any dynamically allocated storage must be released in the same evaluation of constant expression.

(since C++26)
This section is incomplete
Reason: inheritance diagram

Contents

[edit] Members of the primary template

(constructor)
constructs a bad_expected_access object
(public member function)
error
returns the stored value
(public member function)
what
returns the explanatory string
(public member function)

std::bad_expected_access::bad_expected_access

explicit bad_expected_access( E e );
(since C++23)
(constexpr since C++26)

Constructs a new bad_expected_access<E> object. Initializes the stored value with std::move(e).

std::bad_expected_access::error

const E& error() const & noexcept;
(1) (since C++23)
(constexpr since C++26)
E& error() & noexcept;
(2) (since C++23)
(constexpr since C++26)
const E&& error() const && noexcept;
(3) (since C++23)
(constexpr since C++26)
E&& error() && noexcept;
(4) (since C++23)
(constexpr since C++26)

Returns a reference to the stored value.

std::bad_expected_access::what

const char* what() const noexcept override;
(since C++23)
(constexpr since C++26)

Returns the explanatory string.

Return value

Pointer to a null-terminated string with explanatory information. The string is suitable for conversion and display as a std::wstring . The pointer is guaranteed to be valid at least until the exception object from which it is obtained is destroyed, or until a non-const member function (e.g., copy assignment operator) on the exception object is called.

The returned string is encoded with the ordinary literal encoding during constant evaluation.

(since C++26)

Notes

Implementations are allowed but not required to override what().

Inherited from std::bad_expected_access<void>

Members of the bad_expected_access<void> specialization

(constructor)
constructs a bad_expected_access<void> object
(protected member function)
(destructor)
destroys the bad_expected_access<void> object
(protected member function)
operator=
replaces the bad_expected_access<void> object
(protected member function)
what
returns the explanatory string
(public member function)

Special member functions of bad_expected_access<void> are protected. They can only be called by derived classes.

Inherited from std::exception

Member functions

[virtual]
destroys the exception object
(virtual public member function of std::exception) [edit]
[virtual]
returns an explanatory string
(virtual public member function of std::exception) [edit]

[edit] Notes

Feature-test macro Value Std Feature
__cpp_lib_constexpr_exceptions 202502L (C++26) constexpr std::bad_expected_access

[edit] Example

This section is incomplete
Reason: no example

[edit] See also

exception indicating checked access to an optional that doesn't contain a value
(class) [edit]
exception thrown on invalid accesses to the value of a variant
(class) [edit]

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